[ValueTracking] Improve reverse assumption inference

Use isGuaranteedToTransferExecutionToSuccessor() instead of
isSafeToSpeculativelyExecute() when seeing whether we can propagate
the information in an assume backwards in isValidAssumeForContext().
The latter is more general - it also allows arbitrary loads/stores -
and is also the condition we want: if our assume is guaranteed to
execute, its condition not holding would be UB.

Original patch by arielb1.

Differential Revision: https://reviews.llvm.org/D37215

llvm-svn: 368723